Summary: This version adds USB 3.0 support, a equivalent of FUSE for character devices used for proxying OSS sound to ALSA, some memory management changes that improve interactivity in desktops, readahead improvements, ATI Radeon Modesetting support, support for Intel's Wireless Multicomm 3200 Wifi devices, kernel support and a userspace tool for performance counters, gcov support, a memory checker for unitialized memory, a memory leak detector, a reimplementation of inotify and dnotify on top of a new filesystem notification infrastructure, btrfs improvements, support for the IEEE 802.15.4 network standard, IPv4 over Firewire, many new drivers, small improvements and fixes.

1. Prominent features (the cool stuff)

1.1. USB 3 support

This version Linux adds support for USB 3.0 devices (contributed by Sarah Sharp from Intel) and the hardware that implements the eXtensible Host Controller Interface (xHCI) 0.95 specification. No xHCI hardware has made it onto the market yet, but these patches have been tested under the Fresco Logic host controller prototype.

Code: drivers/usb/host/xhci*

1.2. CUSE (character devices in userspace) and OSS Proxy

CUSE is an extension of FUSE allowing character devices to be implemented in userspace, it has been contributed by Tejun Heo (SUSE)

It can be used for many things, for example "proxying" OSS audio from OSS apps through the ALSA userspace layer, or to an audio system which can forward the sound through the network. ALSA contains OSS emulation, but sadly the emulation is in the kernel, behind the userland multiplexing layer, which means that if your sound card doesn't support multiple audio streams (most modern cards don't), only either one of ALSA or OSS emulation interface would be usable at any given moment.

OSS Proxy uses CUSE to implement the OSS interface - /dev/dsp, /dev/adsp and /dev/mixer. From the POV of the applications, these devices are proper character devices and behave exactly the same way, so it can be used as a replacement for the in-kernel ALSA OSS emulation layer. The app sends the audio to these CUSE devices, and the OSS Proxy will forward it to a "slave" (currently there's only one slave implemented, pulseaudio)

1.3. Improve desktop interactivity under memory pressure

PROT_EXEC pages are pages that normally belong to some currently running executables and their linked libraries, they shall really be cached aggressively to provide good user experiences because if they aren't, the desktop applications will experience very long and noticeable pauses when the application's code path jumps to a part of the code which is not cached in memory and needs to be read from the disk, which is very slow. Due to some memory management scalability work in recent kernel versions, there're some (commonly used) workloads which can send these PROT_EXEC pages to the list of filesystem-backed pages (the ones used to map files) which are unactive and can get flushed out of the working set. The result is a desktop environment with poor interactivity: the applications become unresponsive too easily.

In this version, some heuristics have been used to make much harder to get the mapped executable pages out of the list of active pages. The result is an improved desktop experience: Benchmarks on memory tight desktops show clock time and major faults reduced by 50%, and pswpin numbers are reduced to ~1/3, that means X desktop responsiveness is doubled under high memory/swap pressure. Memory flushing benchmarks in a file server shows the number of major faults going from 50 to 3 during 10% cache hot reads. See the commit link for more details and benchmarks.

1.4. ATI Radeon Kernel Mode Setting support

This version adds Kernel Mode Setting (KMS) support for ATI Radeon. Hardware supported is R1XX,R2XX,R3XX,R4XX,R5XX (radeon up to X1950). Works is underway to provide support for R6XX, R7XX and newer hardware (radeon from HD2XXX to HD4XXX).

1.5. Performance Counters

The Performance Counter subsystem provides an abstraction of special performance counter hardware registers available on most modern CPUs. These registers count the number of certain types of hw events: such as instructions executed, cachemisses suffered, or branches mis-predicted - without slowing down the kernel or applications. These registers can also trigger interrupts when a threshold number of events have passed - and can thus be used to profile the code that runs on that CPU. In this release, support for x86, PPC and partial support for S390 and FRV have been added.

Users are not expected to use the API themselves. Instead, a powerful performance analysis tool has been built: "perf", which is available at tools/perf/ (in an unusual decision of including kernel-related userspace software into the kernel tree).

perf supports a few modes of operation, like "perf top", which shows a top-like interface, which you can restrict to any given set of events, process or CPU. There's also "perf record", which records a profile into a file, and "perf report", which reads the profile and shows it in the screen, or "perf annotate", which reads the data and displays the annotated code. There's also "perf list", which shows the list of events supported by the hardware, and "perf stat", which runs a command and gathers performance statistics which are printed into the screen. All the documentation and man pages are available in the 'Documentation' subdirectory. Some examples:

1.7. Gcov support

This version enables the use of GCC's coverage testing tool gcov with the Linux kernel. gcov may be useful for: debugging (has this code been reached at all?), test improvement (how do I change my test to cover these lines?), minimizing kernel configurations (do I need this option if the associated code is never run?) and other things.

1.8. Kmemcheck

Kmemcheck is a debugging feature for the Linux Kernel. More specifically, it is a dynamic checker that detects and warns about some uses of uninitialized memory. Userspace programmers might be familiar with Valgrind's memcheck. The main difference between memcheck and kmemcheck is that memcheck works for userspace programs only, and kmemcheck works for the kernel only.

Enabling kmemcheck on a kernel will probably slow it down to the extent that the machine will not be usable for normal workloads such as e.g. an interactive desktop. kmemcheck will also cause the kernel to use about twice as much memory as normal. For this reason, kmemcheck is strictly a debugging feature.

1.9. Kmemleak

Kmemleak provides a way of detecting possible kernel memory leaks in a way similar to a tracing garbage collector, with the difference that the orphan objects are not freed. Instead, a kernel thread scans the memory every 10 minutes (by default) and prints any new unreferenced objects found in /sys/kernel/debug/kmemleak and warns about them ti . A similar method is used by the Valgrind tool (memcheck --leak-check) to detect the memory leaks in user-space applications.

1.10. Fsnotify

Fsnotify is a backend for filesystem notification. Fsnotify itself does not provide any userspace interface but does provide the basis needed for other notification schemes such as dnotify, inotify and fanotify (this last notification interface, will be included in future releases). In fact, in this release dnotify and inotify have been rewritten in top of fsnotify, removing at the same time the ugly and complex code from those systems. Fsnotify provides a mechanism for "groups" to register for some set of filesystem events and to then deliver those events to those groups for processing, and the locking is much simpler. Fsnotify has other benefits, like shrinking the size of an inode.

1.12. Context Readahead algorithm and mmap readhead improvements

This version introduces a page cache context based readahead algorithm. The current readahead algorithm detects interleaved reads in a passive way, the context readahead algorithm guarantees to discover the sequentialness no matter how the streams are interleaved. The beneficiaries are strictly interleaved reads and cooperative IO processes (i.e. NFS and SCST). SCST benchmarks show 6%~40% performance gains in various cases and achieves equal performance in others

There're also some improvements to mmap readahead. On a NFS-root desktop, mmap readahead reduced major faults by 1/3 and no obvious overheads, mmap io can be further reduced by 1/4.

3. Filesystems

Btrfs

Mixed back reference (FORWARD ROLLING FORMAT CHANGE). It scales significantly better with a large number of snapshots (commit)

Use hybrid extents+bitmap rb tree for free space. Currently btrfs has a problem where it can use a ridiculous amount of RAM simply tracking free space. As free space gets fragmented, we end up with thousands of entries on an rb-tree per block group, which usually spans 1 gig of area. This patch solves this problem by using bitmaps for parts of the free space cache. The maximum amount of RAM that should ever be used to track 1 gigabyte of diskspace will be 32k of RAM (commit)

md: Use new topology calls to indicate alignment and I/O sizes (commit)

14.3. Network

Add new Intel Wireless Multicomm 802.11 driver (iwmc3200wifi). This driver supports Intel's full MAC wireless multicomm 802.11 hardware. Although the hardware is a 802.11agn device, it currently only support 802.11ag, in managed and ad-hoc mode (no AP mode for now) (commit)